text:
What is the best altitude to penetrate a thunderstorm?
135.293(a)(7)(iii) requires that pilots be tested on:

(iii) Operating in or near thunderstorms (including best penetrating
  altitudes), turbulent air (including clear air turbulence), icing,
  hail, and other potentially hazardous meteorological conditions;

So what are the best penetrating altitudes when forced to operate in a thunderstorm?
The most obvious answer is to stay out of it in the first place, but the reg requires us to come up with an answer....

This is one of those regulations that on the surface appears to contradict another one: Intentionally penetrating a thunderstorm strikes me as a flagrant violation of 91.13 outside a few very specific cases like Hurricane Hunters. Looking at it another way though, if a commercial flight inadvertently blunders into a thunderstorm I'd hope my pilot remembers this part of their training :-)

@voretaq7: Well, I don't think that it is a contradiction.  I would assume that this is for penetrating a thunderstorm when you don't have a choice (ie the weather has closed in around you, most likely due to poor decision making in the first place).

Personally I view that poor planning and decision making as the violation of 91.13 - but of course having said that, yes: it behooves you to be familiar with "what to do when it all goes wrong". There are freak pop-up thunder storms & not everyone has in-cockpit weather...

@voretaq7: Haha, yes there are a LOT of people without in-cockpit weather.

Until I spring for the Stratus I'm one of 'em :-)

I thought the best altitude to be at in a thunderstorm was always zero feet.

@DJClayworth: Well, both the question and the answer say to stay out of them, and the other answer explicitly says the same thing, lol.  ;-)

The obvious answer you mentioned should be reiterated: Avoid thunderstorms whenever possible! Just because there are "best penetrating altitudes" doesn't change the fact that thunderstorms are extremely hazardous to aircraft and should be avoided in the first place.
With that said, the FAA's Advisory Circular 00-24C mentions that, if unable to avoid penetrating a thunderstorm:

To avoid the most critical icing, establish a penetration altitude below the freezing level or above the level of -15ºC.

This is to help minimize rapid accumulation of clear ice:

Supercooled water freezes on impact with an aircraft. Clear icing can occur at any
  altitude above the freezing level but at high levels, icing from smaller droplets may be rime or
  mixed rime and clear. The abundance of large, supercooled water droplets makes clear icing very
  rapid between 0ºC and -15ºC, and encounters can be frequent in a cluster of cells. Thunderstorm
  icing can be extremely hazardous.

Another point to mention is that, regardless of there being a recommended "penetration altitude," it is important to maintain a constant attitude, not altitude:

It is almost impossible to hold a constant altitude in a thunderstorm, and maneuvering
  in an attempt to do so greatly increases stress on the aircraft. Stresses are least if the aircraft is
  held in a constant attitude.

But again, emphasizing the dangers of thunderstorms, the circular states:

Never regard any thunderstorm lightly, even when radar
  observers report the echoes are of light intensity. Avoiding thunderstorms is the best policy.

and

Weather recognizable as a thunderstorm should be considered hazardous,
  as penetration of any thunderstorm can lead to an aircraft accident and fatalities to those on
  board.

0 AGL. Do not ever penetrate a thunderstorm in an airplane. It is stupid. People die that way every year.

I agree (and said add much in my question), however that isn't the answer that they are looking for here....

I know. I just wanted to emphasize this. Yours answers the question better.

Is that advice specifically for small planes & general aviation or for everyone?

Everyone, including airliners. Armored research planes may be an exception. https://ztresearch.wordpress.com/tag/t-28-storm-penetrating-aircraft/

I was asked this question on a 135 ride. Stumped for an answer, I was told I will get comfortable flying through if I wanted the job. As this was in the plains, 5000 or 6000 depending on the direction of the flight was the recipe. Not high enough for icing most of the time, yet enough altitude to recover from downdrafts. This also kept us in Approach/Departure airspace and their radar was much better at depicting live weather, as opposed to what the Center controllers handling higher altitudes would be seeing. Asking for vectors through level 1 or 2 was OK but taking level 3 when unavoidable was a rough ride. Using tango prefix in the call sign let them know I was neither a doctor nor an attorney. Night time lowered their workload so they could find a way to get us across squall lines, coming up with diversions around this stuff. After getting my head slammed into the ceiling I learned to hunch when it got bumpy. Few months of that, seeing thundersnow and learning that lightning flashes looks pink when you get close made me go back to flying passengers. Soon after the Check 21 legislation killed that whole business model but for many many years before me guys and gals were doing this every night with no accidents or incidents related to the practice.

Great answer. I too did some work in the plains and across the rockies. The plains have, by far, the worst storms. Thunderstorms can be penetrated but it was always my last resort, and only with onboard radar. I would not attempt it with delayed weather services overlayed on EFB charts.


text:
Is FAA approval required in order to fly an RNAV SID/STAR for a private aircraft operated under FAR Part 91?
I know that under FAR Part 135, specific approval is required for an RNAV SID/STAR (given via an OpSpec), but what about Part 91?  Does the aircraft/pilot have to be approved, and if so how is the approval obtained?

The best answer I can give is generally, no.
In relatively few cases – for aircraft operated under Part 91 Subpart K, which applies to fractional ownership like Netjets – the operator must possess an Mspec, which would be applied for at the local FSDO. I'm not sure what that process looks like, but the FAA's Commercial Operations Branch has some good information on that front.
For all other Part 91 operations the aircraft simply needs to be appropriately equipped.

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text:
What is "Operational Control"?
14 CFR 121 and 135 certificate holders are required to maintain operational control of their flight operations:
§135.77   Responsibility for operational control.
Each certificate holder is responsible for operational control and shall list, in the manual required by §135.21, the name and title of each person authorized by it to exercise operational control.
121.533, 121.535, and 121.537 contain similar requirements for 121 carriers.
What exactly is "Operational Control"?

Operational control is defined in 8900.1 Volume 3, Chapter 25 as:

Operational control functions include, but are not limited to:

Preflight planning;
For part 121 operations, preparation and dissemination of dispatch/flight releases;
Canceling a flight due to potential hazardous or unsafe conditions;
Ensuring that only those operations authorized by the OpSpecs are conducted;
Ensuring that only crewmembers who are trained and qualified in accordance with the applicable regulations are assigned to conduct a flight;
For part 121 operations, ensuring that no flight is started unless it has been authorized by a dispatcher or person authorized to exercise operational control in accordance with §§ 121.593, 121.595, or 121.597, as applicable;
Ensuring that crewmembers are in compliance with flight and duty time limitations and rest requirements prior to departing on a flight;
Designating a pilot in command (PIC) and, where applicable, a second in command (SIC) for each flight;
Providing the personnel who perform operational control functions (PIC, dispatcher, etc.) with access to all necessary information for the safe conduct of the flight (for example, weather, Notices to Airmen (NOTAM), airport aeronautical data (analysis), and inoperable instruments and equipment);
Specifying the conditions under which a flight may be dispatched or released (weather minimums, flight planning, airworthiness of aircraft, aircraft loading, and fuel requirements);
For part 121 operations, ensuring that when the conditions, limitations, and authorizations specified in a dispatch or flight release cannot be met, the flight is either cancelled, delayed, rerouted, or diverted;
For part 121 operations, ensuring that each flight is conducted in accordance with the conditions and limitations of the dispatch or flight release;
Ensuring that each flight is operated in accordance with the limitations of the MEL and CDL;
For part 121 operations, monitoring the progress of each flight and initiating timely actions when the flight cannot be completed as planned, including diverting or terminating a flight;
For part 135 operations, locating a flight for which a Federal Aviation Administration (FAA) flight plan (FP) has not been filed;
For part 121 operations, ensuring rapid and reliable communications in accordance with §§ 121.99 and 121.122, as applicable;
Restricting or suspending operations in accordance with §§ 121.551, 121.553, or 135.69 as applicable; and
Exercising the emergency authority described in §§ 121.557, 121.559, and 135.19, as applicable.

These functions are normally performed by a variety of people:

Dispatch
Crew Scheduling
Maintenance
Chief Pilots
Director of Operations
The PIC of the flight
Manager of SOCC (System Ops Control Center)
Director of Training

Operational control is implemented by the above people and must hit all of the bullets in the definition above.  This means that to exercise operational control you must always know where your planes are, you must determine who flies them, plan those flights, ensure the pilots and aircraft are legal, ensure everyone is trained for the operations performed and to always be able to communicate with your aircraft.
There is also mention of Operational Control Systems, which require that any flight operated by the carrier is dispatched, followed and can be located at all times.  These functions are normally carried out by the Dispatcher assigned to your flight.
Further reading beyond the 8900.1 can be found in AC 120-101 for 121 ops.

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text:
When should an Obstacle Departure Procedure (ODP) be used?
Buried away in the front matter of the FAA charts is a section titled "IFR TAKEOFF MINIMUMS AND (OBSTACLE) DEPARTURE PROCEDURES".  Note that this is not a graphical departure procedure that is in with the rest of the approach plates.
Knoxville, TN (KTYS) contains the following departure procedure in that section:

DEPARTURE PROCEDURE: Rwy 5L, climb heading 048°  to 3400 before
  proceeding on course. Rwy 5R, climb  heading 048° to 3500 before
  proceeding on course. Rwy  23L, climb heading 228° to 3500 before
  proceeding on  course. Rwy 23R, climb heading 228° to 3500 before
  proceeding on course.

Question: If I receive an IFR clearance along the lines of "*N1234, cleared to XXX as filed, climb and maintain...." and I didn't file any kind of departure, am I expected to fly the above ODP, and will ATC know what I'm doing if I do?  Does it matter if it is VMC or IMC?

According to the definition of the FAA it is a recommended procedure, so there is generally no need to follow it.

OBSTACLE DEPARTURE PROCEDURE (ODP)- A preplanned instrument flight rule (IFR) departure procedure printed for pilot use in textual or graphic form to provide obstruction clearance via the least onerous route from the terminal area to the appropriate en route structure. ODPs are recommended for obstruction clearance and may be flown without ATC clearance unless an alternate departure procedure (SID or radar vector) has been specifically assigned by ATC. (Source: FAA Pilot/Controller Glossary)

According to FAR 91.175 you are responsible to ensure sufficient obstacle clearance if the airport is IFR (of course also if the airport is VFR).

(f) Civil airport takeoff minimums. This paragraph applies to persons operating an aircraft under part 121, 125, 129, or 135 of this chapter.
  (1) Unless otherwise authorized by the FAA, no pilot may takeoff from a civil airport under IFR unless the weather conditions at time of takeoff are at or above the weather minimums for IFR takeoff prescribed for that airport under part 97 of this chapter.
  (2) If takeoff weather minimums are not prescribed under part 97 of this chapter for a particular airport, the following weather minimums apply to takeoffs under IFR:
  (i) For aircraft, other than helicopters, having two engines or less--1 statute mile visibility.
  (ii) For aircraft having more than two engines--1/2 statute mile visibility.
  (iii) For helicopters--1/2 statute mile visibility.
  (3) Except as provided in paragraph (f)(4) of this section, no pilot may takeoff under IFR from a civil airport having published obstacle departure procedures (ODPs) under part 97 of this chapter for the takeoff runway to be used, unless the pilot uses such ODPs.
  (4) Notwithstanding the requirements of paragraph (f)(3) of this section, no pilot may takeoff from an airport under IFR unless:
  (i) For part 121 and part 135 operators, the pilot uses a takeoff obstacle clearance or avoidance procedure that ensures compliance with the applicable airplane performance operating limitations requirements under part 121, subpart I or part 135, subpart I for takeoff at that airport; or
  (ii) For part 129 operators, the pilot uses a takeoff obstacle clearance or avoidance procedure that ensures compliance with the airplane performance operating limitations prescribed by the State of the operator for takeoff at that airport.

In this case it seems to be the easiest way to follow the ODP, but it's not binding.
Lnafziger  correctly commented that this is not applicable for part 91 operation. That's why I like to add a part of this Article, which I've found on the AOPA website, to my answer.

[...] We cannot assume that ATC will assign any special routing or altitude based on the ODP. They expect the pilot to fly the procedure if necessary without further clearance. FAA Order 7110.65R, Air Traffic Control--the controller's handbook--states, "If a published IFR departure is not included in an ATC clearance, compliance with such a procedure is the pilot's prerogative." In other words, you have to decide whether you need to follow a specific route or procedure and fly accordingly. Now, we might think this is wrong, dangerous, and dumb, but it is the way procedures are arranged.[...]

According to all this information ATC would expect you to follow this procedure, at least in IMC. If you like to fly it in VMC and it does not disagree with your ATC clearance you are also welcome to do so. If there is any confusion I tend to ask and clarify the situation.
In most cases an ODP says that you should climb on runway heading at least until passing xxx feet before turning in a direction. I would always comply with it and if there is severe CAVOK and ATC then asks me to turn earlier, I would do it, so everyone is happy and on the safe side.

I'd recommend reading this link:
Obstacle Departure Procedures.
Seems to have the answer you're looking for: in short if you are IFR you must fly the ODP unless you meet one of the alternative requirements or ATC advises you otherwise.

Welcome at Aviation.SE! I think this could be a great answer if you would try to expand a bit. Try to have the link as reference only and include more information in your answer so that people don't have to follow the link without knowing what it covers. Just a suggestion.


text:
Why is "heavy braking" recommended in an aircraft with carbon fiber brakes in order to prevent brake wear? That seems backwards!
I get that standard brakes and carbon fiber brakes are made out of two different materials, but some of the procedures are different between them.
For instance, they recommend "heavy" braking on landing (even if it isn't required) in order to prevent brake wear.  Why is that?
What other differences are there between them that a pilot needs to know about before flying an aircraft with carbon fiber brakes?

some info (bottom of page): http://www.boeing.com/commercial/aeromagazine/articles/qtr_03_09/article_05_1.html ?

Some brakes (like many racecar applications) actually require some heat to be put into them before they work. That's why when you buy aftermarket pads for your car they may be recommended for track use only because it wouldn't be safe to require a hot lap in your street car before the brakes work well. I'm not sure if this is a similar case where they require some heat or it could just be the way they wear.

I know nothing about this topic but it's quite an interesting one so I did some Googling and found a detailed Airbus document called "Proper Operation of Carbon Brakes". It includes the following information:

All brake manufacturers highlight the fact that carbon wear is heavily
  affected by brake temperature

This is completely different from steel brakes:

...energy is theoretically not the primary parameter for carbon wear,
  whereas it is the  most important one for steel brakes.

The document goes on to say this:

Generally speaking, to increase carbon brake life, brakes should
  either be operated cold or hot but not at intermediate warm
  temperatures

And this:

All recommended braking techniques should aim at reducing the number
  of brake applications and optimizing the carbon brake temperature

The specific temperatures mentioned in the document are different for different manufacturers, but generally the intermediate range to avoid seems to be roughly 100-300C. That would explain why 'heavy' braking might be recommended: it's better to use one single brake action that generates more heat and gets into the higher temperatures than to brake multiple times more lightly. Nevertheless, the document also says that overheating is still unsafe and the use of max autobrake is not recommended.
I found a related discussion of exactly this question elsewhere that goes into the physical explanation (although it doesn't quote any sources): at lower temperatures, the carbon becomes a powder which abrades the brake pads, lowering its life; as the temperature rises, the carbon starts behaving like a fluid so the wear is greatly reduced. Most discussions of carbon brakes that I found say that their physical properties and behavior are well documented, but not well understood.

The amount of time spent on the brakes is the most important thing which dictates the wear of carbon brakes, not the amount of energy put into them (which is the case with steel brakes).
By braking harder, you are spending less time on the brakes in total, which means that you will have less wear on the brakes. Let's just imaging you wear 1/10000th of the brake every second, no matter how hard or soft you brake. By braking lightly, taking 20 seconds to stop, you'd wear out 1/500th of the brake. But if you only took 10 seconds, you'd wear just 1/1000th of the brake. While it's not quite as simple as that, that's the general idea of it.
Another contribution factor is the non-linear and non-proportional correlation between brake wear and brake temperatures. While this is slightly counter-intuitive, brake wear increases until about 200°C disk temperature (note that indicated temperature is usually lower), then it actually decreases from there until it becomes much hotter (about 700-800°C). So, by braking harder, you will also heat up the brakes to the point where brake wear is less than by braking lightly.
Because of this, it's better to apply the brakes in one go, to reduce the total time on the brakes. Boeing's excellent article on the Operational Advantages of Carbon Brakes gives the following advice when taxiing:

Taxi braking recommendations for carbon and steel brakes
Because the wear mechanisms are different between carbon and steel
  brakes, different taxi braking techniques are recommended for carbon
  brakes in order to maximize brake life.
Steel brake wear is directly proportional to the kinetic energy
  absorbed by the brakes. Maximum steel brake life can be achieved
  during taxi by using a large number of small, light brake
  applications, allowing some time for brake cooling between
  applications. High airplane gross weights and high brake application
  speeds tend to reduce steel brake life because they require the brakes
  to absorb a large amount of kinetic energy.
Carbon brake wear is primarily dependent on the total number of brake
  applications — one firm brake application causes less wear than
  several light applications. Maximum carbon brake life can be achieved
  during taxi by using a small number of long, moderately firm brake
  applications instead of numerous light brake applications. This can be
  achieved by allowing taxi speed to increase from below target speed to
  above target speed, then using a single firm brake application to
  reduce speed below the target and repeating if required, rather than
  maintaining a constant taxi speed using numerous brake applications.
  Carbon brake wear is much less sensitive to airplane weight and speed
  than steel brake wear.
These recommendations are intended as general taxi guidelines only.
  Safety and passenger comfort should remain the primary considerations.


text:
What is the "8900" that the FAA uses for guidance?
The 8900 is sometimes quoted as a reference when trying to determine FAA policies.
What exactly is it and where can it be found?  Is it regulatory in nature?

The Flight Standards Information Management System, created by FAA Order 8900.1, is the source of all information relating to aviation safety inspector job tasks.  It's about 8,000 pages worth of (non-regulatory) guidance and policy helping FAA Inspectors to do their job properly.
It's an electronic-only document, which can be found at fsims.faa.gov, or by visiting the FAA homepage and entering "8900" in the search box at the top right.
There's all kinds of good stuff in there, such as:

How do I give a practical test for a CFI applicant with a sport pilot rating?
How do I perform field approval of a major repair or alteration?
How do I do a ramp inspection?

And lots more!
FSIMS is open to the general public - if you want to know what an inspector does day in and day out (or what to expect when you meet one), FSIMS is the one-stop-shopping location for the answers to all your questions.


text:
How do you correct a mistake on a pilot license?
If a mistake is found on a pilot license, like the pilot's nationality, date of birth, or a missing type rating, how would one go about having it corrected?

I had this happen to me on my first license. I got in touch with the FAA Examiner and he had me contact the local Flight Standards Office (FSDO) who corrected it for me.

If it was a mistake on the application, your Certifying Officer (read: FAA Examainer) can allow you to edit the information through IACRA (https://iacra.faa.gov/iacra/default.aspx) and the license re-issued.
If all of the information was correct, but it was misprinted, you would want to call your local FSDO.
FSDO Search: http://www.faa.gov/about/office_org/field_offices/fsdo/
Your local FSDO can help you with:

Low-flying aircraft
Accident Reporting
Air carrier certification and operations
Aircraft maintenance
Aircraft operational issues
Aircraft permits
Airmen certification (licensing) for pilots, mechanics, repairmen, dispatchers, and parachute riggers
Certification and modification issues
Enforcement of Airmen & Aircraft Regulations

Contact your FSDO.
Changing personal details like date of birth or nationality requires the pilot to visit the FSDO personally.
It's hard to see how a type rating could be missing because completion of the rating should be logged in IACRA and that will trigger the issuance of a new license, so the 'worst' that could happen is that the FAA doesn't issue the new certificate quickly enough.
But obviously sometimes things just go wrong (I had an instrument rating go missing completely, despite IACRA) and in that case you would also follow up with the FSDO to get the certificate re-issued or corrected.


text:
Does the B787 (Dreamliner) operate with higher cabin pressure?
The Boeing 787 Dreamliner's fuselage is almost completely made of composite carbon fibre material, which is not susceptible to metal fatigue.
The main reason why the cabin pressure in a pressurized aircraft is kept as low as possible is to reduce the expansion and shrinking of the fuselage due to changes in pressure differences, reducing metal fatigue in the long run.
Does the 787 use higher cabin pressures than other commercial aircraft? Boeing touted this as one of the revolutionary new features of the aircraft back in 2006, but does it actually use higher cabin pressure now that the plane is in use by airlines?

One thing to note is that while the 787 does operate with a higher cabin pressure (lower cabin altitude) than most airliners in commercial operation, it is not exclusive in doing so. The Airbus A350 also operates with a higher cabin pressure, as do most charter airliners (aka "private jets"). So Boeing describing it as "revolutionary" is only true to a limited extent: admittedly it was the first generally-commercially-operated airliner to have this "feature" (the A350 came later), but was certainly not the first passenger aircraft to do so.

According to this document by Boeing:

Altitude: How High Is Just Right?
Today's airplanes are pressurized to a typical cabin altitude of 6,500
to 7,000 feet (1,981 – 2,133 m), with a maximum certification altitude
of 8,000 feet (2,438 m). Because the advanced composite materials that
make up the 787’s fuselage do not fatigue, the 787 can be pressurized
more, which allows for lower cabin altitude levels.
Studies at Oklahoma State University explored the effect of altitude
on passengers to determine optimum levels. After testing at various
altitudes, it became clear that lowering the cabin altitude to 6,000
feet (1,830 m) provided meaningful improvements. Lowering the cabin
altitude further, however, provided almost no additional benefit.
Based on that knowledge, Boeing designed the 787 to be pressurized to
a maximum cabin altitude of 6,000 feet.

So if we assume a constant altitude of 43,000 (the 787's service ceiling1), we would get a maximum differential of:

6,000 ft Cabin Altitude = 9.06 psi
8,000 ft Cabin Altitude = 8.11 psi

This would indeed be a higher cabin pressure than what Boeing considers "typical" at maximum altitudes.

1 It is probably actually an even bigger difference since the ceiling of a lot of airplanes isn't this high so the max differential would be even lower.

Would that also decrease discomfort during descent for people sensitive to ear/sinus problems?

@PhilippeLeybaert Absolutely!  If the "altitude" in the airplane is lower, that means that it has to descend less in order to get back to the destination airport altitude.  It's the amount of "difference" that affects people, so the lower the cabin altitude the better!

Wasn't one of the other key comfort benefits to having a composite fuselage that cabin air could be at a higher humidity versus in a traditional aluminum fuselaged plane?

@NickT Yes!  With a traditional aluminum airframe, the humidity had to be kept low so that it didn't cause corrosion.  With a composite airframe, the humidity can be higher!

It may surprise you to know that the 6000 feet cabin pressure wasn't even a new feature when the Boeing 787 released! The Aérospatiale-BAC Concorde, the world's first and last profitable supersonic airliner maintained its cabin pressure at 6000 feet AMSL.
From Wikipedia:

Airliner cabins were usually maintained at a pressure equivalent to 6,000–8,000 feet (1,800–2,400 m) elevation. Concorde’s pressurisation was set to an altitude at the lower end of this range, 6,000 feet (1,800 m).[98] Concorde’s maximum cruising altitude was 60,000 feet (18,000 m); subsonic airliners typically cruise below 40,000 feet (12,000 m).

The Concorde's airframe had to deal with several factors that most subsonic aircraft aren't even designed for, including thermal expansion of the fuselage, stress due to contraction when returning to subsonic flight, and the pressure difference at 60000 feet AMSL.
Based on several 787 trip reports, the pax do report that they felt comparatively much better in the 787 after an ultra long haul flight, compared to other contemporary airliners. This is due to the higher cabin pressure and higher artificial humidity maintained in the cabin. The revolutionary feature of the 787 is the bleedless engines (i.e., air for cabin pressurisation is not compressed by the engines, unlike in almost every airliner).

The aircraft does use a lower equivalent altitude in the cabin, while usually operating at an altitude higher than most other aircraft. Since it's designed for it, and I don't think it makes a difference it fuel consumption hence there's no reason to opt out. It's highly appreciated by crew and passengers alike since it leaves them feeling more rested and less jetlagged. It's around 6000 feet rather than the usual 7500-8000 feet.
What also contributes substantially to passenger comfort is the fact that the air is considerably more humid, since corrosion is less of a concern, also leaving passengers and crew dehydrated.
I've never flown on one, but it seems to be holding:

"As for the cabin pressure and moisture, it's all true: I arrived both
  times feeling well-hydrated and without the parched skin that would
  result from flying in any other airplane."
  Source

and a little more on the  workings of this system:

"Cabin – Pressurization differential pressure maximum is 9.4 psid, so
  the cabin altitude is only 6000 feet when at the max cruising altitude
  of 43,000 feet. There is a cockpit humidifier switch, and cabin air
  humidification is fully automatic." Source

and here's an article from 2011:

Most conventional passenger jets set the cabin pressure at an
  equivalent of around 7,500 to 8,000 feet above sea level, which Boeing
  claims is the primary cause of a range of in-flight ills. “There are
  many passengers problems associated with altitude – headaches, muscle
  aches, fatigue and even nausea” Craver says. The difference between
  external air pressure when an aircraft cruises at 40,000 feet and an
  internal pressure that’s one-fifth of that stresses the plane’s
  fuselage – and the greater the difference, the more the stress. That’s
  been the limiting factor in increasing cabin pressure, Craver
  explains: the metal body of current aircraft wouldn’t safely be able
  to handle the fatigue induced by maintaining this pressure at high
  altitudes. That changes due to the use of carbon-fibre composite
  materials on the 787’s fuselage. Carbon-fibre doesn’t suffer from
  metal fatigue and in turn allows for lower 'cabin altitude' levels.
  The 787’s cabin pressure is set to 6,000 feet, a figure arrived upon
  by Boeing modifying a pressure chamber to look like an airplane cabin
  which could hold 12 people at a time. “We cycled over 500 people
  through the chamber, and they stayed there for up to 20 hours of
  simulated flying time” Craver recalls, and they found that 6,000 feet
  was the ‘sweet spot’. “Between sea level and 6,000 feet there was
  almost no difference in the reported symptoms” Craver says, “so we can
  alleviate or mitigate a lot of symptoms you get at a cabin altitude of
  8,000 feet” Boeing claims that one in four travellers experience some
  form of ‘respiratory distress’ after flying 12 hours in a conventional
  aircraft with a cabin pressure of 8,000 feet, but this plummets to 5-6
  per cent at 6,000 feet.
  Source

Lowering the cabin altitude does have an influence on fuel consumption.  It means you are packing more air (and humidity) inside, which increases the weight of the airplaine.

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text:
What exactly is an FBO and what services are provided?
To most non-U.S. pilots who have little or no experience flying in the U.S., the concept of a FBO is not very well understood.
What exactly is a FBO and what are the services that it can provide? I've heard that they can refuel your plane, move it to a hangar, clean it, provide preflight planning facilities, etc.
Is there a charge for these services (I guess so) and if so, what can be expected? I've also seen many airports with multiple FBOs. How does that work?
All I know is that FBO stands for "Fixed Base Operator".
This may seem like a very stupid question but in Europe there's no such thing. If you need fuel you either taxi to the pump and fill up or call up the fuel provider (if you're lucky because they usually only serve private jets).

A Fixed Base Operator (FBO) is an entity - usually a commercial company - which provides "aviation services" at an airport. One of the main services they provide is fueling, either full service or a self service pump.  They don't all provide both 100LL and JetA, but most do.
Larger ones provide other services, such as catering, lavatory service, pre-flight briefing computers and wifi access, ground power units, and tie down or hangar space for parking.  Charges vary from place to place, most large FBOs will charge a fee for aircraft parking, but sometimes waive a day or so of that if you purchase fuel.  They can often assist in arranging ground transport or hotels as well.
Most are open hours like 7AM - 8PM, but that varies as well.  Most self-service pumps can be used 24 hours a day.
Some have a "crew car" available to borrow for a short trip to get lunch or run an errand, though calling ahead to check on that is important.

What if there are multiple FBOs? Can you just pick one and taxi to their ramp after landing? The whole concept just feels rather alien to me :-)

@PhilippeLeybaert Generally if you have a preference for a specific FBO that's the one you'll go to - at a controlled field you would tell ground which FBO you're going to, and at uncontrolled fields you'd taxi over as normal. (Often the preference for one FBO over another is based on how much fuel is going to cost, or if you're making a trip for a vacation somewhere and you've made tiedown arrangements. Many FBOs will also help arrange car rentals for you if you're making a vacation trip.)

It is very useful to chose your FBO during your flight planning or even earlier based on recommendations, special offers or personal preference. You can find the FBOs wich are available at your destination via skyvector or airnav.com. I preferre airnav.com if I'm going to an airport / FBO for the first time, because they offer a airport diagram on wich the FBOs are shown. It's very helpful to know which way you need to taxi in advance, other than asking your self on final wich way you need to vacate the runway - honestly: you can trust me man, I'm talking out of experience ;D

Calling in advance is well recommended.  Some FBOs tend to be more upscale oriented, and eschew small GA aircraft.  Some cater to small GA aircraft and are ill equipped to handle the needs of bizjets.  A phone call also can resolve tiedown and hangering questions.  Most FBOs try to cater to a wide range of customers, and will not only have fuel and human amenities but also maintenance, oxygen service, APU carts, etc.  When flying for work, I tend to gravitate towards 24hr FBOs.


text:
What is a demonstrated crosswind component?
What is a demonstrated crosswind component? Is it possible to land in a crosswind greater than the demonstrated crosswind component? Is it safe to?

I have landed in crosswinds greater than those demonstrated.  The key is to remember to keep the nose straight with rudder and to track the centerline with aileron.  If the wind is strong enough to where have full rudder deflection in and it still isn't enough, it is time to find a different runway.

A demonstrated crosswind component is highest crosswind (corrected to make it 90°) which has been shown to be possible to safely land by a test pilot. It shall not require exceptional skill by an ordinary pilot, however it does not mean every pilot will be able to do so. It is also NOT a limit (contrary to what some say) - if the pilot does decide to land higher crosswinds, they can do so (but please take caution).
This number may not necessarily be the highest crosswind component that it's possible to land in -- it just has to be above the minimum capabilities required in the regulations. For example, say an aircraft, flown perfectly (yes, we're dreaming), was capable of handling a 30 knot crosswind component (no gust, ISA conditions), the manufacturer only needs to demonstrate safe landing with a crosswind component of one-fifth of the reference stall speed in the landing configuration. This would be:

10 knots @ 50KIAS VS0
12 knots @ 60KIAS VS0
14 knots @ 70KIAS VS0
15 knots @ 75KIAS VS0

Reference:

(a) A 90 degree cross-component of wind velocity, demonstrated to be safe for taxiing, takeoff, and landing must be established and must be not less than 0.2 VS0.

I teach a way simpler definition:
The maximum crrosswind component that a plane can hold the fuselage aligned to the centerline up until full deflection of the controls.
Any additional wind requires adding in crab and would result in a side-loaded touchdown which is therefore out if control.

Simpler, but not necessarily more accurate. Max demonstrated is exactly that: demonstrated. Not max possible, but what they had when they demonstrated it. Also, "landing in crab" = "out of control" is utterly wrong. For some aircraft, landing with some crab is both necessary + routine above a certain crosswind component, and not at all "out of control".


text:
Why angle of attack is always shown against the relative wind parallel to horizon?

I have a question about Stall, which I have difficulty to understand.
According to the theory, stall happens when :
1-  Speed is slow then a certain limit.
2-  Angle of attack is greater than a certain limit.
In the case of angle of attack, there is being spoken about relative wind along the wings.
But here is my confusion.
Relative wind is always shown parallel to the horizon, which strikes the wings.
While in my understanding, in the sky there is equal air and wind everywhere. We create our own hard wind for our wings, by moving fast through the air. Right?
So it should not make any difference in which direction we move, along the horizon, or at a steep angle upwards or downwards, relative to the horizon. The relative hard wind will be created straight in opposite direction where we move fast. Because we are also moving in the same direction as our wings angle of attack.
As I have shown with blue line in my illustration above.
If so, then there is no question about angle of attack at all. Because we always create our own wind by moving fast in any direction in the sky.
It would be different if we are moving parallel to the horizon, but only our wings have a greater angle of attack relative to the fuselage and the horizon. (As it is often explained in the example of, hand out of a car. While car is moving horizontally, but only the angle of our hand changes relative to the car). Then this stall theory is understandable.
If we look at the fighter jets and acrobatic airplanes, then we see that they can climb vertically up against the horizon, at 90 degree bank angle and fly upside down too. And they do no stall, because they create their own relative straight opposite wind by moving fast in any direction in the sky.
So why in normal planes we do have to think about straight only horizontally coming relative wind and accordingly the angle of attack against it?

You can drop the number 1 point on your list. Regardless if speed, stall always happens when a certain angle of attack is exceed. Also, the question is very confusing, maybe check out this:  https://aviation.stackexchange.com/questions/2903/how-does-stall-depend-on-angle-of-attack-but-not-speed

This question may be badly phrased, but its meaning is clear. It deserves an upvote.

Unfortunalely, alpha dot, that is the rate of AoA increase, also has an influence on stall. And Mach number. And Reynolds number. And aspect ratio. And wing sweep. It is rather complicated.

I know what the definition of Stall is. But you did not understand my question, I am afraid. Otherwise I did not asked this question here. Never mind, some people did understand my question.

@S.M. Nawaz, Your blue lines would be correct if the airplane were keeping a constant AOA and pitching up to climb.  However, the whole purpose of the pictures is to show increasing AOA in level flight up to the point of a stall.

@PeterKämpf looking at the question I don't think opening that can of worms is justified here.

It is rather simple. Relative wind is ALWAYS exactly equal to the True Airspeed velocity of the aircraft, and flows (relative to the aircraft), in exactly the opposite direction of the aircraft True Airspeed velocity. The diagrams above say absolutely nothing about the Aircraft Velocity, so any assumption about it is problematic. They are drawn that way for simplicity, to IMPLY that the aircraft, and the wing are moving horizontally through the air to the left. The ambiguity would be resolved by adding a True Airspeed velocity vector, labeled as such, pointing horizontally to the left.

Also, the assumption that stall happens when speed is slow is WRONG. stall happens when the AOA exceeds the critical AOA, PERIOD. You can be at 10 knots and not be stalled. You will be accelerating towards the ground, and either falling, or soon to be falling, but if you push the stick/yoke forward enough to "unload" the wing to les than the critical AOA, the wing will not be stalled.

Here's a video with lots of planes flying with their nose pointing up but the plane is moving down: https://www.youtube.com/watch?v=wPanOuWQaT8. The fact that the plane is flying nose up is most obvious when the wheels touch the ground. If the planes were flying nose down the front wheel should touch first. If you observe birds you will also see that they do the same thing when landing on the ground or on tree branches - they start flying in the direction their wings are not pointing at. This is the very core of the concept of angle of attack

Another problem is that on such drawings too much focus is placed on the separation of airflow, perpetuating the myth that airplanes fly because of two neighboring air molecules separated at the leading edge must meet at the end, and the shape of the wing causes the upper one to travel faster, and the Bernoulli principle causes suction to suck the wing upwards... which is obviously false, as planes can fly upside down, and there exist wings with symmetrical cross-sections. The main problem in a stall is that drag starts to exceed lift (and at too much angle of attack lift starts to decrease)

@vsz You're right that the equal transit time thing is bunk, but it's completely true that aerodynamic stall is normally caused by flow separation on the top surface of an airfoil. This is true for symmetric foils as well as asymmetric ones. And the air does indeed accelerate more and produce lower air pressure across the top surface (when not stalled,) again, even in symmetric foils. The definition of aerodynamic stall is an increase in AoA causing a decrease in lift. Flow separation on (part of) the top surface is the normal reason for that to happen.

"the air does indeed accelerate more and produce lower air pressure across the top surface (when not stalled,) again, even in symmetric foils" - yes, that's true, but that's still not where the majority of the lift comes from. The majority of the lift comes from simply the angled wing pushing all that air downwards. But yes, flow separation does reduce (or remove?) that extra lift you mentioned, and it causes turbulence therefore increasing drag, and these two combined drastically reduce the lift.

I think this has been at least hinted at in other answers, but, just to put it succinctly, the diagrams aren't actually intended to show the airflow as always being parallel to the horizon. The airstream is just shown flowing along the horizontal axis of the graph because it's convenient to illustrate it that way. There is no implication intended in the diagram that the horizontal axis of the diagram is actually parallel to the Earth's surface.
The behavior of the airfoil is exactly the same regardless of the actual orientation of the wing and airflow relative to the horizon. All that matters is how the airfoil is oriented relative to the airflow. You can rotate those diagrams any way you want relative to the horizon and what they depict will remain true.
The blue airflow lines you've drawn on the diagrams seem to be assuming that the airplane is always climbing at an angle roughly equal to the angle between the horizon and the wing's chord line, but this is not true. You can fly with a high pitch-up angle while maintaining level flight if you fly slowly enough, for example. You will practice this (called "slow flight") when training for a private pilot certificate. You can (and often do) even descend with a nose-up attitude. And, if you put the nose up too far, you can descend very rapidly with a nose-up attitude after your wings stall. For example, Air France 447 was falling 10,000 feet per minute with a nose-up attitude as one of the pilots held the stick back.

This expands nicely on my answer.

@reirab - Now I began to understand that the angle of attack is not always the flight path. I thought the plane should go wherever its nose is pointed. But apparently it does not happen, if the thrust power is not sufficient to climb up, then the plane goes forward in an angel relative to the upcoming wind. So again a crazy question in my mind. What would be the direction of the relative wind when a plane pitch up its nose and give more thrust to climb up? Would be the relative wind then parallel to the airfoil?

we can add that basic aerodynamics courses may include 2D wind tunnels, where the flow is in fact horizontal, and the wing can be held by the student at different AOA to get a feeling of the forces, similar to the concept that a hand outside the car window.

@S.M. Nawaz: Re "I thought the plane should go wherever its nose is pointed.", have you ever landed a plane?   The nose is definitely pitched up as you descend to touch down on the runway.

@S.M.Nawaz The airflow will almost never be exactly parallel to the airfoil's chord line. That is the definition of zero degrees angle of attack and many wings won't produce any lift at all at 0 degrees AoA. The chord line of the airfoil (an imaginary line stretching from the leading edge to the trailing edge) is almost always inclined at least a couple of degrees relative to the airflow, even in straight-and-level flight. As you add thrust, though, the AoA necessary to sustain a given vertical speed will become more shallow if all else is held equal.

@reirab: Indeed, the AoA is almost always positive, even in a normal descent.  (That is, excluding diving, wingovers, and the like.)  If you observe e.g. airliners on a long final, they'll almost always have a nose up attitude.

@jamesqf Yeah, slight nose-up attitude down final isn't uncommon. Even when the aircraft has a nose-down attitude, though, AoA will still be positive since the airflow will be coming from beneath the aircraft at an angle farther below the horizon than the nose-down angle (and the wings are usually designed with a chord line that is slightly positively inclined even when the pitch angle is zero.) Like you said, the only exceptions to this would be rather extreme maneuvers that are normally only done by fighters, aerobatic aircraft, and the like.

First of all, the relative wind is relative to the airfoil, it has nothing to do with any other direction.
We almost always show the relative wind as horizontal in aerodynamic diagrams, because it is the reference flow for the diagram. Which way up the plane is, or which direction it is moving, does not affect the aerodynamics. All that matters is the relative wind.
The angle of attack is measured relative to this wind.
By the way, slow airspeed is not a condition for stalling, it is a consequence of the stall conditions at low altitude. The stalling speed, at any altitude, is a consequence of the high angle of attack required to maintain lift. The maximum altitude is where the stalling speed and the maximum speed coincide.

I think, relative wind is not coming from any particular direction. But we get relative wind created in any direction we with certain speed. The matter of "angle of attack" would be if wind direction is fixed only horizontally and we are moving against it. But in fact there is almost no wind if we do not move. So why do we see relative wind always parallel to the horizon?

Yes. Which direction the relative wind is physically coming from (the direction of travel), and which direction we draw it in an aerodynamic diagram, are different things.

@S.M.Nawaz If you were making a diagram of angle of attack, which direction would you put the relative wind from?

@HiddenWindshield - I have modified my question above and added a photo, as I think what the relative wind is.

@S.M.Nawaz So, when you draw an AoA diagram, you always draw the wind angle as coming from 25 degrees above horizontal? Ok, that's fine, just a little weird.

@ HiddenWindshield -  I drew blue lines straight on the wing, because the wings are not angled relative to the fuselage. But the fuselage and the wings are heading and traveling in the same direction. Therefore there is NO angle attack.  Sorry if I see it this way.

@S.M.Nawaz In order for the wind to be coming from 25° above the horizon, the airplane would have to be in a 25° climb. Most airplanes don't have enough engine power to sustain such a steep climb for very long. As speed falls, lift is reduced. As lift is reduced, the airplane's climb angle (and therefore the relative wind angle) will fall. If the pilot maintains the same pitch angle, the angle of attack will increase, eventually resulting in a stall.

@S.M.Nawaz Your blue lines are not angled correctly to the airfoil; in the stall, there is no way that air is coming down onto the upper surface! The grey streamlines are in fact the correct direction relative to the airfoil (just as I would draw it). That is why it is called the relative wind and not just the wind.

Look at a typical diagram illustrating angle of attack.
Where on the diagram do you see the horizon?
How do you know the horizon is parallel to the bottom of the picture frame?
It's easy to fall into the trap of thinking that the relative wind is parallel to the horizon, because so many aircraft spend so much time flying so that the relative wind is parallel to the horizon. But in an aircraft that is climbing or descending in calm air, or one that is flying level in a downdraft or updraft, the relative wind is not parallel to the horizon.
If you like, you can rotate the entire diagram on the page. No matter which way you point it, as long as you have the same angle between the relative wind arrow and the chord of the wing, it is the same angle of attack, as illustrated on
this page.
The whole point of angle of attack is that when figuring out the lift of an aircraft's wings, it does not matter where the horizon is. What matters is the interaction between the surrounding air and the wing, which usually depends on the relative motion of these two things, including the direction of that motion.
It is true that in most cases the relative wind around an aircraft has much more to do with the propulsion and control of the aircraft than with anything else. But you are mistaken if you think that an aircraft, even an extremely high-performance jet fighter, will always travel in exactly the direction in which the centerline of the fuselage is pointing.
Consider an air show in which one jet fighter is flying "straight up" and another is flying level. Can you tell, just by looking at it, that the fuselage of one aircraft is at exactly a 90 degree angle to the other fuselage? Or that the flight path is exactly 90 degrees from the other flight path?
Are you 100 percent sure the "straight up" fighter is not actually flying a path 2 degrees away from vertical, while its fuselage is angled 1 degree on the other side of a vertical line? Small angle differences like that are significant when we're talking about angle of attack.
And since you bring up aerobatic aircraft, they often move in directions that are not "straight forward" in the direction the fuselage is pointing.
Many aerobatic maneuvers involve stalls (high positive angle of attack)
and flying upside down usually involves a negative angle of attack.

There is no difference between "normal" planes and "acrobatic" planes as far as stall behavior. If the angle of attack exceeds the critical angle, the wing will stall, period. This is true for any airplane.
I think your confusion lies in this idea that the chord line of the wing is always parallel with the wind flow. That is wrong. As the airplane slows, lift is reduced due to the lack of airflow over the wing. Thus, the pilot has to increase the angle of attack to increase lift to maintain level flight. The opposite is true of increasing airspeed: in fact, some airplanes actually have to maintain a negative angle of attack in cruise to avoid climbing through too much lift. The weight of the plane and the amount of G force it's experiencing are also factors.
In your question, you wrote, "It would be different if we are moving parallel to the horizon, but our wings have a greater angle of attack relative to the horizon." You say that like it's not true, but it is in fact pretty much the definition of level flight.

"There is no difference between "normal" planes and "acrobatic" planes as far as stall behavior." This is really not true. Stall behavior varies quite a lot between even different aircraft within ones that would be considered "normal," let alone between normal, stable aircraft and ones designed for high maneuverability like aerobatics aircraft or fighters. On most airplanes, flow doesn't just separate from the entire foil all at once. Different parts of the airfoil stall sooner or later than others and how this happens has large impacts on stall behavior.

@reirab Yes, but as far as what the OP was asking, which is about the very basics of stall, there's no difference.

S.M. Nawab, your question demonstrates an intuitive understanding of “relative wind” which is something that many others don’t always get right away.  So, your root question is:

“why in normal planes do we have to think about straight coming relative wind and accordingly the angle of attack against it?”

The short answer is that we DON’T have to think about it that way.  If you understand relative wind you already have a leg up on understanding that a stall can happen at any airspeed and attitude relative to the horizon.
The reason that relative wind is depicted as horizontal in most instructional diagrams is because “normal planes” spend most of their time in straight and level flight.  This makes it easier to teach the concept of Angle of Attack to beginning students.
By the time you get to the point where you might need to worry about stalling at 3Gs while inverted at the top of a loop you should have this basic concept mastered!
ADDENDUM:
Don’t take this the wrong way, but I think you have allowed your intermediate level comprehension of relative wind in high performance aircraft and unusual attitudes get in the way of understanding a beginner level illustration.  There is more than enough material here and in the other answers to put you on the right path to understanding, but I just have a couple pointers that my help you out in the future:

Always make sure you understand the learning objective of whatever picture, graph or chart you are looking at.  Read all accompanying text related to the picture!  Context is very important. This very standard illustration is showing how increasing AOA leads to airflow separation and eventually a stall.  It would not accomplish that point if you made an unfounded assumption that the aircraft was adding power and pitching up to climb.

Make sure you are clear on what is being held constant, and what is a variable.  In this illustration altitude is assumed to be constant, (therefore relative wind) and AOA the variable, and it is increasing to create additional lift as the aircraft slows down.  If the relative wind were to change as you assumed, AOA would be shown as a constant.

So I have to consider that relative wind is always coming horizontally parallel to the ground. And we are changing our angle of attack into that wind? While I think that there is (almost) no wind. The relative wind we create ourselves for our wings, by moving fast in any direction. So our relative wind should always come exactly from opposite direction of our heading. Because only our movement cause the wind.

@S.M. Nawaz. I'm not sure what your question is... (or whether you understand the point in my answer)  You are correct that the aircraft creates relative wind by its motion through the air.  Therefore it comes opposite to the direction of travel.  It is depicted as horizontal for simplicity.

My point is: in the air there is no wind, or not enough useful wind to fly in. So when we move through the air upwards in a steep angle or even straight vertical, the wind we get is our relative wind. Which is coming exactly from the opposite direction, because of our motion in that direction. But we still assume that relative wind is coming horizontally and parallel to the earth surface. Thus according to that our angle of attack is determined. As if we are dependent of that imaginary horizontal wind. While our relative wind is ever changing in whatever direction we go as whole with wings.

@S.M. Nawaz, everything you are saying about relative wind is 100% correct. Except that when you say that “we still assume that relative wind is coming horizontally and parallel to the earth surface” you are incorrect. I don’t assume that, and apparently you, (correctly) don’t assume it either. So who is the “we” you describe? I presumed you were referring to a simplified diagram in a flight manual, but maybe you need to explain where you are getting this incorrect assumption from.

@S.M.Nawaz - Question. You keep mentioning the “wind” as in the breeze you feel standing on the ground. Relative wind has nothing to do with the weather wind speed and direction. Relative wind is the direction and intensity (or opposite reaction of) the aircraft moving in the airmass. The fact that the airmass itself is moving can be considered as inconsequential. Unless the aircraft is sitting motionless on the ground.

@S.M.Nawaz - Relative wind is opposite of the aircrafts 3-dimensionally path or track of flight regardless of its attitude. If the aircrafts path of flight is straight up, straight down is the direction of its relative wind. You can extrapolate the same point to a banking aircraft. Relative wind is depicted as horizontal to simplify the imagery of a complex subject. It is similar to the fact that most maps and sectionals are printed with North at the top regardless of which direction you are actually facing. If you are confused, just turn the paper to orient it correctly.

@S.M.Nawaz - Don’t make the ground or the horizon your frame of reference for relative wind. Don’t even use the direction of flight based on the aircraft’s longitudinal axis. Make your frame of reference the path of flight of the chord line of the airfoil through the airmass. The way the airmass hits or the air molecules interact with the airfoil/wings determine the Angle of Attack. Sticking your hand out of a window of a moving car gives your hand an angle of attack. So does sticking your hand out of and airplane in flight or a skydiver on a jump.

@Dean F -  I think, sticking hand out of car cannot be compared to the wings of a plane. Because in a car we change the angle of our hand relative to the body of the car. But in a plane when we change our direction of move then wings and fuselage goes together in that direction. We are making NO angle relative to anything, except the horizon. So in the told theory it feels like that we are moving in the air-mass which lies horizontal to the earth surface, and every directional movement in it would make an angle against it. That's why I dew blue lines to show what I understand of relative wind.

@S.M.Nawaz - Actually, sticking your hand out of a car is the given example because changing your hands angle relative to the movement of the car does not change the hand’s direction nor speed. It only changes the Angle of Attack. I also gave the example of the skydiver changing the angle of their hands. Regardless of the change, their direction of travel is roughly straight down. Just like an airplane can drop roughly straight down, in a flat attitude, if it stalls. Even at full power. This is why Angle of Attack is considered as the important factor in a stall instead of airspeed.

@S.M.Nawaz - Do some research on Power-on Stalls and Accelerated Stalls (stalls caused by increased load factor in a turn or bank attitude). Remember, the direction in which you point the nose of the aircraft is not always the direction in which it is going to fly. Do some research on Slow Flight and Uncoordinated Turns. That is why the car’s direction of travel is irrelevant. Only the Relative Wind created by the hand’s movement through the airmass relative to the hand itself is relevant to Angle of Attack. Even when the car is climbing a hill and not level with the horizon.

@S.M. Nawaz, please see the addendum to my answer above.

@S.M.Nawaz, Consider a dive bomber in a 60 degree dive. His flight path through the air is tilted 60 degrees to the horizon. What direction is the relative wind?

Thank you for your question. Your most recent edit points to where your misunderstanding stems. Relative Wind is a function of flight path relative to the airfoil chord line. It is how the airflow interacts with the wing surface. Your frame of reference is the airfoil, not the ground. Align the Relative Wind with the flight path of the airfoil/wing. For big picture understandings, we can consider this roughly the same as the flight path of the aircraft with a little consideration given to how the wing is angled on the fuselage (Angle of Incidence).
It is the same concept whether the Relative Wind/Flight Path were parallel, perpendicular, or at an angle to the ground. It is true whether the aircraft is flying horizontally, vertically, inverted, straight up, or straight down. This is even true in a loop. The flight path (therefore the Relative Wind) would be roughly the tangent of the loop. Or, even if the airfoil/wing is spinning in circles in front of, above, or to either side the aircraft. Do some research on P-factor, auto-rotation, and vortex ring state. After all, propellers and rotors are airfoil with chord lines and angles of attack.
You are assuming that the aircraft’s flight path changes with attitude. Your blue arrows all represent the Relative Wind if the aircraft airfoil chord lines were relatively parallel to its flight path. That is only true in the first diagram. In the second and third diagrams, the chord line of the airfoil is not parallel to the flight path. The flight path in all three diagrams is from left to right, parallel to the top and bottom of the page.
In the first diagram, the airfoil is flying level into the Relative Wind created by the movement of the aircraft through the airmass. In the second and third diagram, the aircraft’s flight path has not changed. The airfoil’s position in the Relative Wind has changed. This could be caused by a sudden change in attitude. The aircraft’s flight path would not change until aerodynamics and power plant thrust overcomes the aircraft’s momentum. It could also be caused by the reduction of power necessitating an increase in nose up pitch in order to maintain level flight. For instance, pulling back on the control yoke abruptly and violently would change your attitude before it would change your flight path. Also, doing straight and level slow-flight would cause you to fly with your nose in an abnormally nose high pitch attitude.
To use your example of fighter planes and acrobatic planes, let’s look at some real world examples. If you ever watch a fighter plane rapidly change pitch, you will notice fog or clouds form just aft of the leading edge of the wing. This visible moisture is not visible when the aircraft is flying straight and level in unaccelerated flight. It only happens when the aircraft abruptly changes attitude. No matter how it looks, the aircraft’s change in flight path is not as abrupt as its change in attitude. In the case of an abrupt pitch nose up, the angle of attack will abruptly change until the aircraft’s flight path realigns itself to the new attitude.
Don’t make the ground or the horizon your frame of reference for Relative Wind. Don’t even use the direction of flight based on the aircraft’s longitudinal axis. Make your frame of reference the path of flight of the chord line of the airfoil through the airmass. The way the airmass hits or the air molecules interact with the airfoil/wings determines the Angle of Attack.
Relative wind is opposite of the aircrafts 3-dimensionally path or track of flight regardless of its attitude. If the aircrafts path of flight is straight up, straight down is the direction of its Relative Wind. You can extrapolate the same point to a banking aircraft. Relative wind is depicted as horizontal to simplify the imagery of a complex subject. It is similar to the fact that most maps and sectionals are printed with North at the top regardless of which direction you are actually facing. If you are confused, just turn the paper to orient it correctly.
Try this with your diagrams with your blue lines removed. If you are flying straight and level in unaccelerated flight, the first diagram would represent your Angle of Attack. If you were to reduce power to slow your airspeed while maintaining the same attitude, you would start to descend in altitude. The second diagram would represent this if you turn it to keep the airfoil chord line parallel to your frame of reference (the actual ground) in your visual field. If you were to bring the power to idle while maintaining the same attitude, you would descend at a faster vertical speed. The third diagram would represent this if you were to turn it so that the airfoil chord line remains parallel to your frame of reference (the actual ground) in your visual field.
To understand this from the frame of reference of the airmass, think of yourself as a human shaped air molecule either stationary or moving at a different speed or direction than the airfoil. Your feet are pointing toward the earth and your head toward the sky. If the airfoil from the previous paragraph strikes you while in level flight like in diagram one, the leading edge would hit you right in the gut. If the airfoil were to strike you while in a descent or in nose-up attitude level flight at a slower airspeed like in diagram two, the underside of the airfoil would hit you in the forehead. If you increase the descent rate while maintaining a level attitude or increase nose up pitch in straight and level slow flight like in diagram three, more of the underside of the airfoil would hit you on the top of your head.

Now I began to understand that the angle of attack is not always the flight path. I thought the plane should go wherever its nose is pointed. But apparently it does not happen, if the thrust power is not sufficient to climb up, then the plane goes forward in an angel relative to the upcoming wind. So again a crazy question in my mind. What would be the direction of the relative wind when a plane pitch up its nose and give more thrust to climb up? Would be the relative wind then parallel to the airfoil?

@S.M.Nawaz - Close. The Relative Wind is always roughly the flight path of the airfoil which is almost the same as the flight path of the aircraft. The flight path relative to the airmass is not always the direction in which the nose of the aircraft is pointing. The aircraft’s flight path will depend on attitude control as well as power measured in engine thrust and/or airspeed (you need at least one if not both of these). Remember, Lift makes an aircraft climb, descend, or remain at constant altitude. Lift overcomes gravity. Thrust overcomes drag regardless of attitude.

Only at an extreme nose up pitch attitude does engine thrust contribute enough to overcoming gravity for it to make lift not a factor. As airspeed slows (which will be the case in nose up pitch), lift decreases. No airspeed, no lift. And, lift is always perpendicular to the airfoil chord line. But, with sufficient airspeed, you can continue to climb even without thrust as power. The Angle of Attack is the difference between the relative flight path through the airmass of the airfoil and the chord line of the airfoil. It does not necessarily rely on thrust. Think of a glider.

During Slow Flight, your pitch is nose up. You can ascend, descend, remain at constant altitude, or turn. Below a certain speed threshold and under a certain power setting, altitude is controlled by power. Airspeed is controlled by pitch. “Pitch for airspeed. Power for altitude.” is a pilot mantra when flying on the backside of the power curve. On the front side, changing pitch to a nose up attitude will make the aircraft climb. The relative wind will even eventually align itself with the airfoil chord line, given enough airspeed and/or power. But, you will rapidly begin to lose airspeed.

It's quite a bit of a simplification, but I can't see anyone pointing this in the other answers:
If your plane is not climbing or descending (level flight) then your motion will be parallel with the horizon and so will be the relative wind (albeit in oposite direction).
If the wing (well, plane) was in a climb then that blue arrow should indeed be pointing a little bit down (depending on your speed and rate of climb, google Flight Path Angle).
If the wing is in a descent, relative wind would indeed have a slight upward component.
This would only happen if you choose the horizon to be your frame of reference (ie. Horizon is always horizontal and on the X axis of our graph. Vertical is always on Y axis). In practice you can choose to have the direction of motion as your X axis, and Y axis just something perpendicular to that motion. In this second case the relative wind will always be "horizontal" because that is how you decided to draw the axes.
Note that in first case (horizon is X axis) the wing will have to be rotated at it's corect pitch angle, while in second case (flight path is X axis) the wing will be rotated only by the aoa. Second is a bit simpler. And second is what most graphics will show

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text:
1.VRB02KT: Is it varying more or less than 180°? 2.VRB03KT: Is it surely varying more than 180°?
Based on the following ICAO Annex 3 criteria for the use of the abbreviation VRB (not FAA, which is different and evident):

when the total variation is 60° or more and less than 180° and the wind speed is 3 kt or more, such directional variations shall be reported as the two extreme directions between which the surface wind has varied

when the total variation is 60° or more and less than 180° and the wind speed is less than 3 kt, the wind direction shall be reported as variable with no mean wind direction; or

when the total variation is 180° or more, the wind direction shall be reported as variable with no mean wind direction"

I bring up the following QUESTIONS

Based on these criteria, is there a way to identify if a variable 2KT wind is varying less or more than 180° when the indication is just VRB02KT?

If it were varying less than 180° but at least 60° or more, would it be possible to measure its mean direction and direction range despite its low intensity of just 2KT? E.g. 15002KT 110V200.
If not, it brings me back to my first question, what is the actual variation of a VRB02KT, less or more than 180°? Could I just answer impossible to specifically tell, but it is surely 60° or higher, otherwise the mean direction would be possible to measured, even at only 2KT. E.g. 15002KT

If the wind intensity following a VRB indication is 3KT or more, can I assume it is varying more than 180°? Since this excessive variation in direction (not the relatively low intensity of 3KT) is the reason why the station can't measure the mean direction precisely.

E.g. VRB03KT would mean varying by more than 180° (not 60° or more, and less than 180°) with an intensity of 3KT (which would be strong enough to provide feasible conditions for measuring the mean direction, if it wasn’t for its excessive variation in direction above 180°).

It doesn't, but my question is based on these criteria, because of its inconclusiveness when considering the indication VRB02KT. I was asked this question by an instructor and neither of us could answer.

Pragmatic answer: It doesn't matter. The wind velocity is too small to have any operational impact.

I know the pragmatic answer, but some instructors don't like it. That's why I'm looking for a confirmation. Thanks for your answer!

@Marcos, The pragmatic answer is that you don’t have to worry about it because you will never hear VRB02.  Instead, you will hear “winds calm” or “light and variable” if less that 3kts.  In any case, the effects are inconsequential so don’t overthink things.

Based on these criteria, is there a way to identify if a variable 2KT wind is varying less or more than 180° when the indication is just VRB02KT?

No, there is no way to tell. And you don't need to tell. The wind velocity is too small to have any operational impact.

If the wind intensity following a VRB indication is 3KT or more, can I assume it is varying more than 180°? Since this excessive variation in direction (not the relatively low intensity of 3KT) is the reason why the station can't measure the mean direction precisely.

Yes, that is safe to assume, as per the definition you quoted.
Remember the values you see in a METAR are average values over a period of time. If you need the immediate wind, ask the tower.

Thanks for your answer. That's exactly what I thought but decided to look for a confirmation, either from experts or other pilots.


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What is the most severe engine failure for modern turbofan?
Today I read again something about the Southwest Airlines Flight 1380 and the related engine failure resulting in one passenger being sucked out of the window. As I understood it was a fan blade out incident and normally the engine are certified for it.
Are the engines certified for all failure possible ?
If not what would be the most sever engine failure possible on modern engines ?

A good and interesting question!  @MaximEck's answer has a lot of good points, which I will not repeat, but rather try to complement.
My credentials for this answer: I formerly worked at a large engine OEM and was personally involved in the certification of many different engine models.
First, I want to clarify the definition of "certified".  I think there may be an implicit assumption in the question, and maybe MaximEck's answer too, that "certified" is synonymous with "tested". It is not.  "Certified" merely means that the manufactured has proved to the FAA (or EASA, or whatever regulatory agency) that the engine complies with all the pertinent regulations.  For some regulations, the manufacturer might show compliance by a full engine test, but for other regulations it might be by a component test, an analysis, or comparison to another certified engine.
For FAA, the necessary regulations are 14 CFR part 33 (https://www.ecfr.gov/cgi-bin/text-idx?node=pt14.1.33&rgn=div5)
For example, part 33.62 Stress analysis reads

A stress analysis must be performed on each turbine engine showing the design safety margin of each turbine engine rotor, spacer, and rotor shaft.

So, are rotor disks certified against disk burst? Yes they are. But they are certified by analysis, not necessarily by test.  Now, the analysis here is extremely detailed.  There may be a team of multiple people that spend years (years!) running the analysis to prove to the FAA that a given disk design is okay.
The other thing to clarify is a "contained" failure versus an "uncontained" failure. I think MaximEck was trying to get at this but said "certified" instead of "contained". A fan blade failure, if it does occur, is required to be contained, (and it is generally required to demonstrate that containment by test):

§33.94   Blade containment and rotor unbalance tests.
(a) Except as provided in paragraph (b) of this section, it must be demonstrated by engine tests that the engine is capable of containing damage without catching fire and without failure of its mounting attachments when operated for at least 15 seconds, unless the resulting engine damage induces a self shutdown, after each of the following events:
(1) Failure of the most critical compressor or fan blade while operating at maximum permissible r.p.m. The blade failure must occur at the outermost retention groove or, for integrally-bladed rotor discs, at least 80 percent of the blade must fail.

There is no such containment requirement for disks (the simple reason is that containing a disk failure would require a structure so heavy that the airplane would not be able to fly... or at least not in any economically feasible way).
Now, we know, from MaximEck's answer that although extremely rare, it is not unheard of for disks to fail and bladeout events to be contained. There can be many reasons for this and I will not go into it here.  But the fact that this does occasionally occur does not mean that the engines weren't certified to all applicable regulations.
To address some of the other points:

Are the engines certified for all failure possible ?

No. There are a number of possible failure conditions that are not covered by any regulations.  These are, however, so exceedingly rare that you generally do not need to worry about them, although they are admittedly "possible".
For example, military engines are often required to pass a "live fire" test (https://apps.dtic.mil/dtic/tr/fulltext/u2/a528013.pdf), showing that they can survive even if you shoot bullets at them, but commercial engines are not. You can conceive of many possible failure scenarios when shooting at an aircraft, but since that is very rare for commercial engines, it's not covered.

If not what would be the most sever engine failure possible on modern engines ?

Fan blade out is generally considered to be the most severe contained failure possible on large commercial turbofan engines, with rotor disk burst being the most severe unconstrained failure.

"Failure of the most critical compressor or fan blade" I'm no jet engine expert, but aren't all the blades pretty darn critical? An unbalanced compressor or fan is a Bad Thing™ as they're not likely to be balanced with one missing. As a guy who was involved in this for many years, what does this really mean, since my take is obviously simplistic and uneducated?

@freeman, for a high bypass turbofan, the fan blades are MUCH bigger than all of the other blades, by one to two orders of magnitude. If you loose a compressor blade, yes that that will cause some unbalance. On something like a 777, the centrifugal unbalance force of one blade out (at redline speed) is about 350,000 pounds, which is more than half of the max takeoff weight of the plane (~750,000).

Ah, so that's "Failure of the most critical compressor blade or fan blade"... i.e. there are 1 or 2 compressors stages that are critical and fall into this section along with the fan and its blades. Correct? I'm sure this makes perfect sense to those well versed in the field, but I'm not that guy.

@FreeMan, yes if you meant circumferential direction, then yes they are all equal.  The regulation is refer to stages. For a high bypass turbofan, usually the high pressure compressor and turbine stages are analyzed with computer simulation, but not tested.  The fan is the most critical stage and is tested.  For a low bypass turbo fan, the difference between the fan and 1st compressor stage is a lot smaller, and you may or may not determine the fan to be the worst case failure.

I think I'm more-or-less getting it now. Thanks for being patient...

Thanks for the answer, yes I misused the word certified instead of tested in order to gain certification and not just numerical analysis and simulations.

Here are my initial finding so far, and I will add information that I find along the route:
There seems to be two types of engine failures. Tested and untested ones. The ones that the engine are certified for through test are the ones that are likely to happen in the fleet life. And therefore, the engine manufacturer must show that those kinds of failure will not put the life of the passenger at risk. The other are unexpected incident that shouldn't have happened and are certified through numerical analysis.
The most extreme scenario in the tested failure category seems to be the fan blade out. The unbalance created by losing a fan blade creates huge vibrations and breaking torque when the fan rubs against the casing. In addition to that, the compressor is stalled so you have backward flow in the engine and huge ball of fire coming out. (see this video).

To obtain the engine certification, the manufacturer must demonstrate during a dedicated test that all broken blades remain in the engine and that no heavy parts will exit either in direction of the aircraft or fall to the ground.
Sadly, it doesn’t work as expected and recently two CFM56-7B have had FBO event that didn’t end as well as planned.
Southwest Airlines Flight 3472 and 1380.

Now the untested failure options. Two different examples came during my search, rotor disc rupture and engine loss. Both should never happen but cannot be mitigated if they happen and that's why they cannot be certified.
Let’s start with the American Airlines Flight 191, which engine number one was lost during takeoff. Whereas it is not the engine that failed but the fixations, the consequences where tragic.
Secondly the rotor disk rupture. This time it is not the blades (fan turbine or compressor) that fell off but the whole disk retaining them that fails. This seems mostly due to manufacturing defect with crack propagation not detected during maintenance. The kinetic energy of those disk is such that it would require a tank shield to prevent them from escaping from the engine which weight would be horrendous. Sadly, they are more example of those:
Delta Air Lines Flight 1288 (MD-88, the NTSB determined the most probable cause of the accident was a fracture in the left engine's front compressor fan hub)

National Airlines Flight 27 and United Airlines Flight 232 (DC-10 Tail engine fan disk rupture leading to flight control losses, loss of cabin pressure and passenger sucked out)

Qantas Flight 32 (A380, turbine disk disintegrated, causing extensive damage to the nacelle, wing, fuel system, landing gear, flight controls, engine controls, and a fire in a fuel tank that self-extinguished)

Air France 88 (A380, Fan disk failure. The examination determined that the hub failure was caused by a low cycle fatigue cracking process which originated in the part’s subsurface. 21 month later a 150kg piece of the engine was located 4 meters below the snow and was recovered in Greenland.)

If severity in your context requires doing major damage to the airframe, then this Air France A388 flight does not qualify, even if the engine failure is catastrophic: http://avherald.com/h?article=4af15205/0008&opt=0

Btw, the Aviation Herald article has some nice pics, always nice to have nice pics...

Na that definitely does count to me... a 2.5m fan falling from the sky should not happen... And everyone loves nice pics ^^

One can imagine the bang associated with the fan hub splitting like in this case... they were damn lucky the parts missed the airframe as they departed. The ingredients for a total loss were definitely there...

" A 150kg piece of the engine was located 4 meters below the snow and was recovered." Imagine that rotating at 3000RPM and boom going through the wing or the main frame... Very lucky indeed. But that is kind of correlated to the increase of safety, every incident we witness is a major one as all small flaws have been eliminated through testing and certification. (except 737 max, but I think it is gonna be the safest plane when it will resume flying)

@Jpe61 If a propeller blade detaches, there is nothing at all to contain it, unlike a turbofan where the engine casing will always take out some of the energy in the debris. I once saw a nice picture of a C130 which had lost a prop blade. There was a nice blade-shaped hole in the exit door on one side of the fuselage where the blade went in, and a similar hole in the exit door on the other side where it came out again. Luckily, the accident happened on the ground and there was nobody inside at the time.

@MaximEck the chance of a detached fan hitting the wing is near enough zero. When the fan shaft breaks, the fan is still generating thrust but that is now acting on 150kg of fan disk, not many tonnes of airframe. The result is that the disk has an initial acceleration of 100g or more forwards from the wing and is a long way in front of the aircraft before its motion starts to become unstable/unpredictable.

@Jpe61 It would be correct if the fan shaft broke, the fan would keep spinning away from the engine. But in the case of the fan hub or rotor disc (older engine) failing, the fan instantaneously stop rotating and all parts fly away in a straight line. From there it's hard to say whether the blades will still generated some lift or not. (citation of the BEA below). Considering the propeller blades not being enclosed you are right, but they also spin slower which means lower kinetic energy and also they might create less debris with thus lower chance of impacting other aircraft vital systems ?

@Jpe61 "The failure occurred during a normal operational phase (cruise climb) without the appearance of any precursory event. The radial (upward and downward) expelling of the two fan hub high-energy fragments at the time of the separation was random. A lateral ejection (left and right) of the same debris might have had catastrophic consequences, for instance if the airframe had been pierced or if the wing spar, control surfaces or flight control systems had been damaged." BEA analyse

@alephzero wrong tag sorry...

@MaximEck, just to add some information about the south west failure. The blade that ruptured was actually contained by the casing. So regardless of further damages, this failure falls under the certification scope (of the engine). What caused the further damages are the high vibration levels stemming from the blade hitting and rubbing against its casing, that led to a failure of the inlet cowl itself. This parts falls in the nacellist or even the aircraft manufacturer scope, not the engine manugacturer, and is not designed to for blade containment. Nice answer nonetheless

@MaximEck, more generally, I would say the worst engine failure would be a disk failure. Engines can be designed to withstand blade off, but no engine is likely to stop a failing disk from going through its casing. There is simply too much kinetic energy in the disk. That is why on some turbine disks, blades are designed to fail at some speed to avoid critical overspeed leading to an impossible to contain failure of the disk

Please ensure pictures you use are public domain and correct sourced, otherwise they risk being deleted.

To paraphrase the above answers, the most serious failure is an uncontained failure.
The fan receives most attention for certification. The shroud is supposed to contain a failure of the fan. Much effort goes into this.
However it's possible that the failure mode may be so serious that the shroud doesn't contain the failure. SWA1380 was an example of this as was UA232. Other examples exist.
In the case of UA232, the failure was traced to a defect in the titanium billet used to manufacture the fan that went undiscovered for years until it failed.
Whilst the fan receives most attention (for good reasons) other failures can happen.
In the case of British Airtours 28M, a combustor can inside the engine failed, causing a fire during the takeoff roll. Although the flight crew reacted promptly and even 'parked' the plane near to firefighters, many lives were lost. This was a turbojet engine rather than a turbofan but the failure mode exists for either type.
In the instance of Quantas QF32 the failure wasn't of a fan disk but a turbine disk. An incorrectly manufactured oil pipe failed and sprayed oil into the engine's 'hot section'. The resulting fire overheated the disk which then disintegrated and likewise caused an uncontained failure.


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How does a large airport switch flow?
Airplanes prefer to land into the wind, and airports adjust for changing wind direction by changing the active runway.
I've been flying a small plane in the pattern when the tower asked me to do a 180° turn to reverse the pattern.
But at a large airport with traffic continuously coming-n-going, with fast moving jets that fly enormous patterns, how do they switch directions?   It seems logistically complicated.

Probably let incoming flights already in the close patterns finish their approach and lead new flights into the new flow.

I suppose you mean when there is traffic on approach, landing and/or taking off. Otherwise the switching time doesn't affect any aircraft.

I mean that at a major Class B airport such as ATL, CHI, SFO, they pretty much have landings and departures non-stop.  How do they safely have aircraft taking off and landing cross-ways to each other, given that big jets probably spend 5+ minutes on final.

The key is holding patterns.  Busy airports have charted "arrival procedures" which tell inbound IFR planes to follow a series of named points called "fixes" or "waypoints".  At any of these points, ATC can instruct a plane to "hold", which means they fly a racetrack pattern passing over that point again and again until released.  This essentially stops the inbound traffic from coming any closer.  Note that multiple planes can be "stacked" over the same point at different altitudes, so just a few points can hold dozens of planes if needed.  (For low-traffic airports, which often don't have arrival procedures, holds may not be needed at all; ATC can just give inbound planes a few extra vectors to kill time.)
If the destination airport needs to change runways, any plane already past the last holding point will (usually) be allowed to continue, but all new arrivals will be put in a hold.  Once the last plane is down on the old runway(s), ATC starts releasing planes from the holds to continue on to the new runway(s).  For a while, new arrivals will still get put in a hold while earlier ones are being released, but eventually the stacks empty out and planes are once again flying the entire arrival procedure without any holds along the way.
These same holding patterns are also used when bad weather limits the arrival rate of the airport due to increased separation requirements.  If that lasts long enough for the holding stacks to start getting full, other ATC units will start holding planes at points further and further away--possibly on the ground at their origin if needed--or even diverting them to other airports.

It's not too logistically complicated on its own.  You just stop takeoffs, start taxiing the conga line of waiting airplanes to the new runway, let airplanes currently on final land, and start vectoring the other airborne flights toward the new approaches once the last of takeoff traffic is away.  Once the new landing flow is established, start releasing the takeoffs.  This will require coordination between tower and approach and may involve the center if flights need to be put in holding during the transition (depends on how busy the airspace is).  In certain busy airspace this may also require coordination with other nearby airports.
With that said, some airports that take off and land on different runways can have impacts on arrival rates in different configurations, and this adds to the logistic complication.  One example is KIAH, which in normal flow is taking off on the 15's and landing on the 26's and 27.  It isn't too much trouble to switch landing traffic to the 8's and 9, but if takeoffs have to be switched to 33 then the flow rates are negatively impacted.  It gets even worse if landing and takeoff traffic have to be put on the same runways.  How the arrival and departure flows are impacted (if at all) from a runway switch will depend strongly on the specific geometry of the runways.
Anecdotally, the logistic impact of reduced arrival and departure flow is probably the largest impact.  At KIAH they will run their normal flow until the wind gets so bad that pilots start refusing clearances for tailwind limitations, and only then will they switch the departures to northbound.  Similarly, as soon as the tailwind is legal, they'll switch back to southbound departures.

For KATL, I've noticed that at some point (the reason for the timing isn't obvious) inbound flights will begin "circling" in oval tracks 100-200 statute miles away while the remaining flights land and depart. Meanwhile outbound begin queuing to takeoff at the "other end" of the taxiway. They seem to finish the switch within 10-15 minutes.

The idea of an orderly airport directional reversal is appealing, and it probably happens if the wind direction changes slowly enough.
However, once I was a passenger flying out of Chicago O'Hare (ORD) and was fifth or sixth in line to takeoff after maybe ten minutes of "waiting" (slowly taxiing) in a long line of aircraft.  Suddenly the plane is braked to a full stop and the pilot announced—quite annoyed sounding—that the airport is being turned around and we will be taxiing around the airport for awhile, sorry for the inconvenience, we'll get you on your way soon, yadda, yadda, yadda.
I wish I'd had an airport diagram to see what the situation was, but we taxied for a full 55 minutes and crossed several runways.  My sense of direction suggested that we had gone nearly full circle around the tower.  (In case it makes any difference, this was in a cold February.)